arXiv subjects

Adam K. Leibovich

Publications and source records attributed to Adam K. Leibovich.

At least 55 records · Page 3Linked to original sources

arXiv2003

Comment on Quark Masses in SCET

Quark masses are included in the SCET Lagrangian. Treating the strange quark mass as order Lambda_{QCD}, we find that strange quark mass terms are suppressed in SCET1, but are leading order in SCET2. This is relevant for B decays to K^* and K. Strange quark mass effects in semileptonic and weak radiative form factors are studied. They give corrections to the form factors that are not suppressed by powers of the bottom quark mass, or, equivalently, by the large recoil energy of the final state meson, and and preserve the heavy to light form factor relations that follow from using the leading order current.

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arXiv2002

The Resummed Photon Spectrum in Radiative Upsilon Decays

We present a theoretical prediction for the photon spectrum in radiative Upsilon decay including the effects of resumming the endpoint region, E_γ-> M_Υ/2. Our approach is based on NRQCD and the soft collinear effective theory. We find that our results give much better agreement with data than the leading order NRQCD prediction.

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arXiv2002

Radiative Upsilon Decay at the Endpoint

The standard NRQCD power counting breaks down and the OPE gives rise to color-octet shape functions at the upper endpoint of the photon energy spectrum in radiative Upsilon decay. Also in this kinematic regime, large Sudakov logarithms appear in the octet Wilson coefficients, ruining the perturbative expansion. Using SCET, the octet shape functions arise naturally and the Sudakov logarithms can be summed using the renormalization group equations. We derive an expression for the resummed octet energy spectrum.

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arXiv2002

Exclusive Semileptonic Decays of B Mesons to Orbitally and Radial Excited D

In this paper we compute, within in the context of a relativistic quark model, the Isgur-Wise functions for exclusive semileptonic $\bar B \to X_c$ decays, where $X_c$ is any charmed mesons with total spin $J=0,1,2$ or one of their first excited states. The relevant matrix elements are computed by a direct numerical integration, in coordinate space, of the convolution of the wave function of the $B$ meson at rest and the wave function of the $X_c$ meson, boosted according with its recoil factor. Our results are compared with other predictions found in the existing literature.

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arXiv2002

Increased Yield of ttbb at Hadron Colliders in Low-Energy Supersymmetry

Light bottom squarks and gluinos have been invoked to explain the b quark pair production excess at the Tevatron. We investigate the associated production of ttbb at hadron colliders in this scenario, and find that the rates for this process are enhanced over the Standard Model prediction. If light gluinos exist, it may be possible to detect them at the Tevatron, and they could easily be observed at the LHC.

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arXiv2002

Deconstructing 5-D QED

We discuss periodic compactification and latticization of a 5-D U(1) theory with a Dirac fermion, yielding a 1+3 effective theory. We address subtleties in the lattice fermionic action,such as fermion doubling and the Wilson term. We compute the Coleman-Weinberg potential for the Wilson line which is finite for N-branes >= 3, due to the Z_N symmetry, which replaces translations in the 5th dimension. This mode becomes a PNGB in the low energy 1+3 theory. We derive its anomalous coupling to the "photon" and its KK-modes.

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arXiv2002

Natural Theories of Ultra-Low Mass PNGB's: Axions and Quintessence

We consider the Wilson Line PNGB which arises in a U(1)^N gauge theory, abstracted from a latticized, periodically compactified extra dimension U(1). Planck scale breaking of the PNGB's global symmetry is suppressed, providing natural candidates for the axion and quintessence. We construct an explicit model in which the axion may be viewed as the 5th component of the U(1)_Y gauge field in a 1+4 latticized periodically compactified extra dimension. We also construct a quintessence PNGB model where the ultra-low mass arises from Planck-scale suppressed physics itself.

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arXiv2002

Enhanced subleading structure functions in semileptonic B decay

The charged lepton spectrum in semileptonic $B\to X_u \ell\barν$ decay near maximal lepton energy receives important corrections from subleading structure functions that are formally suppressed by powers of $Λ_{QCD}/m_b$ but are enhanced by numerical factors. We investigate the series of higher order terms which smear over a region of width $ΔE_\ell \sim Λ_{QCD}$ near the endpoint the contributions proportional to $δ(E_\ell - m_b/2)$ times (i) the matrix element of the chromomagnetic operator, and (ii) four-quark operators. These contribute to the total rate at the few percent level, but affect the endpoint region much more significantly. Implications for the determination of $|V_{ub}|$ are discussed.

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arXiv2001

Top quark associated production of topcolor pions at hadron colliders

We investigate the associated production of a neutral physical pion with top quarks in the context of topcolor assisted technicolor. We find that single-top associated production does not yield viable rates at either the Tevatron or LHC. tt-associated production at the Tevatron is suppressed relative to Standard Model ttH, but at the LHC is strongly enhanced and would allow for easy observation of the main decay channels to bottom quarks, and possible observation of the decay to gluons.

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arXiv2001

Resumming the color-octet contribution to radiative Upsilon decay

At the upper endpoint of the photon energy spectrum in Upsilon -> X gamma, the standard NRQCD power counting breaks down and the OPE gives rise to color-octet structure functions. Furthermore, in this kinematic regime large Sudakov logarithms appear in the octet Wilson coefficients. The endpoint spectrum can be treated consistently within the framework of a recently developed effective field theory of collinear and soft particles. Here we show that within this approach the octet structure functions arise naturally and that Sudakov logarithms can be summed using the renormalization group equations. We derive an expression for the resummed energy spectrum and, using a model lightcone structure function, investigate the phenomenological importance of the resummation.

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arXiv2001

A Comment on the Extractions of V_{ub} from Radiative Decays

We present a model independent closed form expression for |V_{ub}|^2/|V_{tb} V_{ts}^*|^2, which includes the resummation of large endpoint logarithms as well as the interference effects from the operators $O_2$ and $O_8$. We demonstrate that the method to extract |V_{ub}| presented by the authors in hep-ph/9909404, and modified in this letter to include interference effects, is not just a refinement of the method introduced in hep-ph/9312311. We also discuss the model dependence of the latter proposal. Furthermore, we show that the resummation is not negligible and that the Landau pole does not introduce any significant uncertainties.

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arXiv2001

Opening the Crystalline Color Superconductivity Window

Cold dense quark matter is in a crystalline color superconducting phase wherever pairing occurs between species of quarks with chemical potentials whose difference δμlies within an appropriate window. If the interaction between quarks is modeled as point-like, this window is rather narrow. We show that when the interaction between quarks is modeled as single-gluon exchange, the window widens by about a factor of ten at accessible densities and by much larger factors at higher density. This striking enhancement reflects the increasingly (1+1)-dimensional nature of the physics at weaker and weaker coupling. Our results indicate that crystalline color superconductivity is a generic feature of the phase diagram of cold dense quark matter, occurring wherever one finds quark matter which is not in the color-flavor locked phase. If it occurs within the cores of compact stars, a crystalline color superconducting region may provide a new locus for glitch phenomena.

hep-ph↗

arXiv2000

Power counting and effective field theory for charmonium

We hypothesize that the correct power counting for charmonia is in the parameter Lambda_QCD/m_c, but is not based purely on dimensional analysis (as is HQET). This power counting leads to predictions which differ from those resulting from the usual velocity power counting rules of NRQCD. In particular, we show that while Lambda_QCD/m_c power counting preserves the empirically verified predictions of spin symmetry in decays, it also leads to new predictions which include: A hierarchy between spin singlet and triplet octet matrix elements in the J/psi system. A quenching of the net polarization in production at large transverse momentum. No end point enhancement in radiative decays. We discuss explicit tests which can differentiate between the traditional and new theories of NRQCD.

hep-ph↗

arXiv2000

|V_{ub}| From Semileptonic Decay and b -> s gamma

Current errors on |V_{ub}| are dominated by model dependence. For inclusive decays, the model dependence comes from the Fermi motion of the b quark. By combining the endpoint photon and lepton spectra from the inclusive decays B -> X_s gamma and B -> X_u l nu, it is possible to remove this model dependence. We show how to combine these rates including the resummation of the endpoint logs at next to leading order. The theoretical errors on |V_{ub}| on the order of 10% are possible. We also give a brief discussion on comparing different extractions.

hep-ph↗

arXiv2000

NRQCD Analysis of Bottomonium Production at the Tevatron

Recent data from the CDF collaboration on the production of spin-triplet bottomonium states at the Tevatron p \bar p collider are analyzed within the NRQCD factorization formalism. The color-singlet matrix elements are determined from electromagnetic decays and from potential models. The color-octet matrix elements are determined by fitting the CDF data on the cross sections for Upsilon(1S), Upsilon(2S), and Upsilon(3S) at large p_T and the fractions of Upsilon(1S) coming from chi_b(1P) and chi_b(2P). We use the resulting matrix elements to predict the cross sections at the Tevatron for the spin-singlet states eta_b(nS) and h_b(nP). We argue that eta_b(1S) should be observable in Run II through the decay eta_b -> J/psi + J/psi.

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arXiv2000

Extracting |V_{ub}| from the Hadronic Mass Spectrum of Inclusive B decays

Following a strategy introduced earlier by the authors, we show that it is possible to extract |V_{ub}| from the cut hadronic mass spectrum of B decays without large systematic errors which usually arise from having to model the Fermi motion of the heavy quark. We present a closed form expression for |V_{ub}|/|V_{ts}| which is accurate up to corrections of order α_s^2, α_s ρ, (Λ/m_b)^2/ρ, where ρis the experimental cut (s_c/m_b^2) on the hadronic mass used to veto charmed decays. Modulo duality violation errors, which are intrinsic to all inclusive predictions, we estimate the theoretical error in the extraction to be at the 5% level.

hep-ph↗

arXiv2000

On the Resummed Hadronic Spectra of Inclusive B Decays

In this paper we investigate the hadronic mass spectra of inclusive B decays. Specifically, we study how an upper cut on the invariant mass spectrum, which is necessary to extract V_{ub}, results in the breakdown of the standard perturbative expansion due to the existence of large infrared logs. We first show how the decay rate factorizes at the level of the double differential distribution. Then, we present closed form expressions for the resummed cut rate for the inclusive decays B -> X_s gamma and B -> X_u e nu at next-to-leading order in the infrared logs. Using these results, we determine the range of cuts for which resummation is necessary, as well as the range for which the resummed expansion itself breaks down. We also use our results to extract the leading and next to leading infrared log contribution to the two loop differential rate. We find that for the phenomenologically interesting cut values, there is only a small region where the calculation is under control. Furthermore, the size of this region is sensitive to the parameter \barΛ. We discuss the viability of extracting V_{ub} from the hadronic mass spectrum.

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arXiv1999

Extracting V_{ub} Without Recourse to Structure Functions

We present a closed form expression for |V_{ub}|^2/ |V_{tb} V_{ts}^*|^2 in terms of the endpoint photon and lepton spectra from the inclusive decays B -> X_sγand B -> X_u\ellν, respectively, which includes the resummation of the endpoint logs at next to leading order and is completely independent of the B meson structure function. The use of this expression for extracting V_{ub} would eliminate the large systematic errors usually incurred due to the modeling of the heavy quarks' Fermi motion.

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